In a projection exposure apparatus used in a photolithography process for the manufacturing a semiconductor device, a liquid crystal display device, a thin-film magnetic head or the like, a circuit pattern with fine and narrow line width formed on a mask (e.g., a reticle) is projectionally exposed onto a photosensitive substrate (e.g., a wafer; hereinafter, a “wafer” is used an exemplary substrate for the description below) with a resist layer applied thereon, via a projection optical system with a high resolution. A wafer stage is moved vertically (i.e., upward and downward) along the optical axis of the projection optical system to align the wafer surface on the wafer stage to the focal position of the projection optical system. The wafer stage is then moved in specific directions two-dimensionally on a plane orthogonal to the optical axis of the projection optical system. For example, in a step-and-repeat type of projection exposure apparatus, the wafer stage is stopped at such a position that the center of the exposure field (i.e., the optical axis) of the projection optical system coincides with the center of each shot region on the wafer, where the exposure operation is performed. In such a projection exposure process, a photosensitive resin (e.g., a novolak resin) is usually applied on the wafer as the resist layer.
Recently, in such a type of apparatus, for addressing the demand for a more miniscule circuit patterns, the resolution of the apparatus has been improved. For this purpose, a light with a short wavelength in the UV zone has been generally used as the exposure illumination light. It has also been demanded to improve the workability of the projection exposure apparatus, which depends on the exposure time period. For this purpose, an exposure illumination light with a high illuminance has been used to shorten the exposure time period. Therefore, for pattern transfer, the surface of the substrate (e.g., a wafer, a glass plate) applied with a photosensitive agent (e.g., a photoresist) is irradiated with a light with a high energy density.
When a light with a high energy density is irradiated onto the surface of the substrate applied with a photosensitive agent, so-called “ablation” may occur in which the photosensitive agent or a reduction product thereof (hereinafter, referred to as a “photosensitive agent or the like”) is scattered or vaporized and then evaporated. During the projection exposure process, the photosensitive agent released from the substrate partly reaches onto the optical member disposed in the vicinity of the substrate, and deposited on the surface opposed to the substrate (hereinafter, simply referred to as the “substrate-opposed surface”) of the optical member, sometimes resulting in contamination of the surface of the optical member.
The deposits (e.g., a photosensitive agent or the like) on an optical member exhibit the behavior of optical contamination on the optical member. For example, in the case of an optical member as a component of a projection optical system, at the time of pattern transfer, the illuminance on an area of the substrate surface (applied with a photosensitive agent) corresponding to the contaminated area of the optical member may be decreased compared to that corresponding to the non-contaminated area of the optical member. That is, regional unevenness in amount of exposure light may occur in the pattern transferred onto the substrate surface applied with a photosensitive agent. As a result, for example, the line of a pattern designed to have an even line width may be transferred unevenly. For the manufacture of an integrated circuit pattern, such uneven pattern line transfer may ultimately cause functional failure of the finished product.
In addition, when the deposits on the optical member can absorb the illumination light, the temperature of the optical member is increased as the deposits are heated by the absorption of the illumination light during pattern transfer. Consequently, the optical characteristics of the optical member are altered, ultimately resulting in the alternation of the total imaging characteristics of the entire projection optical system.
In order to overcome this defect, in a projection exposure apparatus, it has been conventionally required to wipe the surface of an optical member of a projection optical system by an operator manually (cleaning process). It has also be proposed to provide a replaceable cover glass or polymer film between an optical member and a wafer to thereby prevent the approach of contaminants to the optical member (Japanese Patent Application Laid-open No. 6-140304).
It is the recent trend to increase the numerical aperture (NA) of a projection optical system to the maximum for the purpose of increasing the resolution of the system to the maximum. In this case, the maximum of the incident angle of the exposure illumination light upon the wafer is more increased. Then, it has been proposed to bring the projection optical system close to the wafer at the time of pattern transfer, thereby decreasing the diameter of the optical system of the exposure apparatus. This technique is also beneficial for the reduction in aberration for optical design of the exposure apparatus.
However, when the projection optical system is brought closer to the wafer at the time of pattern transfer, the deposition of contaminants (e.g., a photosensitive agent and the like) are released from the wafer onto the optical member of the projection optical member. Thus, the contamination of the optical member in the projection optical system is a serous problem. Moreover, the use of an exposure illumination light with a shorter wavelength and a higher illuminance for the purpose of improving resolution and workability of the apparatus makes this problem more serious.
In a prior art technique for disposing a replaceable contamination protection member between a projection optical system and a wafer, the space between the projection optical system and the exposure surface of the wafer has a three-layered structure, at the time of pattern transfer, which consists of a layer of the atmosphere between the projection optical system and the contamination protection member, a layer of the contamination protection member, and a layer of the atmosphere between the contamination protection member and the exposure surface of the wafer. Therefore, there is a limitation in bringing the projection optical system close to the wafer at the time of pattern transfer.
In addition, because of the increased demand for high transfer accuracy, the installation repeatability for a replaceable contamination protection member and the permissible optical variability of the contamination protection member become very critical. Therefore, the application of a cover glass (which has an even thickness) as the replaceable contamination protection means has its limit. That is, when a contamination protection member is replaced with a new one, it is inevitable to re-adjust the total aberration balance of the projection optical system; which is not suitable for practical use.
When a polymer film is used as the replaceable contamination protection member, the re-adjustment of the aberration balance of the projection optical system is not necessary. However, among the components of the exposure illumination light, the transmittance on the film of components having a large incident angle upon a polymer film are decreased. Particularly, the transmittance of an S-polarized light (which is largely responsible for an imaging characteristic) is remarkably increased, resulting in poor imaging performance. Moreover, a polymer film is generally difficult to be applied with an anti-reflection coating; which is not also suitable for practical use.
Under these circumstances, the present invention has been accomplished. The first object of the projection exposure apparatus according to the present invention, therefore, is to provide a projection exposure apparatus with improved precision by reducing the contamination caused by the deposition of a photosensitive agent or the like on an optical member of a projection optical system or the like.
The second object of the projection exposure apparatus according to the present invention is to provide a projection exposure apparatus with improved accuracy, by cleaning an optical member of a projection optical system or the like at the time when pattern transfer is not performed.
The third object of the projection exposure apparatus according to the present invention is to provide a projection exposure apparatus with improved accuracy by inspecting an optical member of a projection optical system or the like for contamination and, based on the inspection result, cleaning or replacing the optical member at the time when pattern transfer is not performed.
The fourth object of the projection exposure apparatus according to the present invention is to provide a projection exposure apparatus with improved accuracy by protecting an optical member of a projection optical system or the like for contamination during pattern transfer is performed.
The object of the projection exposure method according to the present invention is to provide a projection exposure method for performing the pattern transfer operation satisfactorily under the condition where the contamination caused by the deposition of a photosensitive agent or the like on an optical member of a projection optical system is reduced.